The general purpose of this research is to understand how embryonic cells, all descended from the same zygote, become determined for different developmental fates. The unfertilized egg plays a major role in the initiation of cell determination in Drosophila. We are attempting to identify and isolate components of the Drosophila egg that act as embryonic determinants. Both messenger-RNAs and proteins of the egg are being examined. In addition, we are isolating maternal-effect mutants in which the eggs produced by homozygous mutant females are defective and cannot support normal development. The possibility that some of the mutant defects might involve egg determinants is under investigation.